1. Field of the Invention
The invention relates generally to voltage controlled oscillators, and more particularly to systems and methods for increasing the frequency ranges achievable by interpolative voltage controlled oscillators.
2. Related Art
Data processing and communication systems often rely on timing signals to enable their operation. For example, clock signals may be used to interpret binary signals as series of ones and zeros, to synchronize the propagation of data through logic circuits, and so on. Clock signals are typically generated by phase locked loop (PLL) circuitry which makes use of voltage controlled oscillators (VCO's). VCO's are devices which produce oscillating signals, where the frequencies of oscillation are dependent upon control voltages which are input to the VCO's.
VCO's are typically constructed using a circularly connected series of inverters. The output of each inverter serves as the input to the next inverter in the series, with the output from the last inverter being routed back to the input of the first inverter. The series includes an odd number of inverters, so that a signal which is initially input to one of the inverters is inverter and odd number of times before being returned to the input of that inverter. Thus, when a high signal propagates through the series of inverters and returns to its starting point, the signal is low, causing a transition which then propagates through the inverters. This process repeats itself, causing the signal at each point in the loop to repeatedly transition between high and low values.
In a simple VCO, the frequency at which the signal oscillates is dependent upon the voltage (the control voltage) which is supplied to the inverters. The higher the voltage, the more quickly the inverters will transition between high and low values, resulting in higher frequencies of oscillation. The lower the voltage which is supplied to the inverters, the more slowly the inverters will transition, resulting in lower frequencies of oscillation. Thus, the lowest frequency which will be generated by the VCO corresponds to the lowest control voltage (e.g., 0,) while the highest frequency corresponds to the highest control voltage (e.g., Vdd, or the power supply voltage.)
In order to increase the frequency of the signal that can be generated by a VCO, interpolative VCO's have been developed. In an interpolative VCO, a series of inverters are circularly connected as described above, but additional circuitry is provided to forward the oscillating signal in the loop, essentially shortening the loop of inverters. Because there are effectively fewer inverters in the loop, the frequency of oscillation increases. The forwarding circuitry between two modes of the loop may be as an inverter and a transistor connected in series. When it is desired to increase the frequency of oscillation, the transistor is switched on in order to forward the signal. When the transistor is switched off, the series of inverters operates in the same manner as the simple VCO described above. While such interpolative VCO's are beneficial in that they enable the generation of higher-frequency signals by effectively shortening the loops of inverters, it would be desirable to provide VCO's which are capable of generating an even wider range of frequencies.